Method of operating fuel cell and power supply system

ABSTRACT

Methods of operating a fuel cell and a power supply systems are provided. The methods and systems of the present invention are capable of suppressing condensation of moisture obstructing an air passage without the need of any complicated system configuration. The method of operating a fuel cell includes controlling a supply of air while monitoring a relative humidity of exhausted air. The step of controlling the supply of air can be performed such that the relative humidity of exhausted air becomes equal to or less than a specific humidity. The power supply system including, as a power supply, a fuel cell having an air supply unit, an air exhaust unit, and a fuel gas supply unit includes a humidity sensor provided on the air exhaust unit, and a control mechanism for controlling a supplied amount of air in the air supply unit based on the information provided from the humidity sensor.

CROSS REFERENCE TO RELATED APPLICATIONS

This patent application is a divisional of U.S. patent application Ser.No. 10/398,339, filed on Apr. 25, 2003, which claims priority toJapanese Patent Document No. P2001-239976 filed on Aug. 7, 2001, thedisclosure of which is herein incorporated by reference.

BACKGROUND

The present invention relates to a method of operating a fuel cell so asto generate an electromotive force by chemical reaction between oxygenand a fuel gas (for example, hydrogen), and to a power supply systemincluding the fuel cell as a power supply.

Fuel cells are adapted to generate an electromotive force fromgenerators by chemical reaction between hydrogen representative of afuel gas and oxygen (air) supplied thereto. In general, each fuel cellhas a structure that an electrolyte film (proton conductive film) issandwiched between gas electrodes, to generate a desired electromotiveforce by chemical reaction generated therein. Such fuel cells aregreatly expected to be applied to electric cars or hybrid vehicles, andfurther, studies are now being made to improve the fuel cells suitablefor use in new applications other than the above-mentioned applicationthereof mounted on vehicles such as cars, for example, in a portableapplication where the fuel cells are incorporated in portable equipment,by making use of advantages of the fuel cells in terms of ease forreduction in weight and size as compared with the existing dry cells andchargeable batteries.

In each fuel cell, moisture is generally produced at a cathode electrodeby chemical reaction between oxygen and hydrogen. The moisture thusproduced may be condensed into a liquid, and the liquid tends tooverflow in an oxygen gas passage, to obstruct diffusion of oxygen gas,thereby inhibiting effective reaction. This leads to a problem inreducing the output of each fuel cell.

The kinds of moisture present in a fuel cell may include, in addition tothe moisture produced by reaction between hydrogen and oxygen, moisturewhich has been contained in a humidified hydrogen side gas and has beenthen permeated on the cathode side through an electrolyte film togetherwith protons, moisture originally contained in the air intake, andmoisture initially present in the form of dew condensation in a flowpassage. To prevent the reduction in output of the fuel cell, it isrequired to exhaust the above-described kinds of moisture obstructing anair passage; however, a large scale system is required to detect all ofthese kinds of moisture and optimally control the exhaust thereof, andtherefore, a need exists to realize detection of these kinds of moistureand optimal control of the exhaust thereof by a small-sized andinexpensive system.

SUMMARY

The present invention provides a method of operating a fuel cell. Morespecifically, the present invention provides a method of operating afuel cell, which is capable of suppressing condensation of moistureobstructing an air passage without the need of a complicated systemconfiguration, and to provide a power supply system using such a fuelcell.

In an embodiment, the present invention provides a method of operating afuel cell generating an electromotive force from the fuel cell bysupplying air and a fuel gas thereto. The method includes the steps ofmonitoring a relative humidity of exhausted air and controlling a supplyof air. The present invention, in an embodiment, also provides a powersupply system including, as a power supply, a fuel cell having an airsupply unit, an air exhaust unit, and a fuel gas supply unit. The powersupply system includes a humidity sensor provided on the air exhaustunit, and a control mechanism for controlling a supplied amount of airin the air supply unit on the basis of information given from thehumidity sensor.

In a fuel cell, moisture may be internally produced due to variouscauses, with the amount thereof varying depending on the conditions ofthe fuel cell. Further, along with the change in temperature in a fuelcell depending on the environment and/or the operational conditionthereof, the saturated water vapor pressure may be significantly varied.According to an embodiment of the present invention, the condensationstate of the water vapor in the fuel cell can be estimated by monitoringa relative humidity at the temperature of the fuel cell. In anembodiment, the temperature of exhaust air and the relative humidity ofexhausted air in a fuel cell correspond to the temperature and thehumidity in the fuel cell, respectively. In this regard, the time pointat which condensation of moisture in the fuel cell begins can beestimated by monitoring the temperature of exhaust air and the relativehumidity of exhausted air in the fuel cell, and thus, condensation ofmoisture can be suppressed by adjusting supply of air based on themonitored result such that the humidity of the exhausted air ismaintained at a specific humidity or less for effective operation of thefuel cell.

To this end, in an embodiment, the present invention provides a methodof operating a fuel cell generating an electromotive force from the fuelcell by supplying air and a fuel gas thereto. The method includes thesteps of monitoring a relative humidity of exhausted air, andcontrolling a supply of air.

In an embodiment, the step of controlling the supply of air is performedsuch that the relative humidity of exhausted air is maintained at ahumidity level effective for operation of the fuel cell.

In an embodiment, the step of controlling the supply of air is performedsuch that the supply of air is increased to an amount effective todecrease the relative humidity of exhaust air thereby allowing effectiveoperation of the fuel cell.

In an embodiment, the method of operating the fuel cell includeshumidifying the fuel gas and supplying the humidified fuel gas to thefuel cell.

In an embodiment, the present invention provides a method wherein thefuel gas includes hydrogen.

In a further embodiment, the present invention provides a power supplysystem having an air supply unit, an air exhaust unit, and a fuel gassupply unit wherein the power supply system includes a fuel cell as apower supply. The power supply system includes a humidity sensorprovided on the air exhaust unit, and a control mechanism forcontrolling a supplied amount of air in the air supply unit based oninformation generated from the humidity sensor.

In an embodiment, the air supply unit includes a pump for supplying air.

In an embodiment, the fuel gas comprises hydrogen.

In an embodiment, the power supply system further includes a currentdetecting unit for detecting an output current of the fuel cell whereina control voltage applied to the pump is outputted from the controlmechanism based on an output current value detected by the currentdetecting unit.

In an embodiment, the power supply system further includes a humidifierfor humidifying a fuel gas to be supplied to the fuel cell andcontrolling the humidified amount of the fuel gas wherein the humidifieris provided on the fuel gas supply unit.

Additional features and advantages of the present invention aredescribed in, and will be apparent from the following DetailedDescription of the Invention and the figures.

Additional features and advantages are described herein, and will beapparent from, the following Detailed Description and the figures.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is an exploded perspective view showing an example of a fuel cellaccording to an embodiment of the present invention.

FIG. 2 is a schematic sectional view showing an example of electrodescontained in the fuel cell according to an embodiment of the presentinvention.

FIG. 3 is a schematic diagram showing an example of a fuel celloperating system according to an embodiment of the present invention.

DETAILED DESCRIPTION

The present invention generally relates to fuel cells. Moreparticularly, the present invention relates to methods of operating fuelcells and power supply systems that include fuel cells to supply power.

A basic configuration of a fuel cell as a basic unit of a fuel cell andan electromotive force generating mechanism will be first described.FIG. 1 shows one configuration example of a fuel cell. As shown in FIG.1, the fuel cell, in an embodiment, is configured by superimposing afuel electrode 1 to an air electrode 2 via an electrolyte 3, to form asub-assembly, and sandwiching the sub-assembly between two currentcollectors 4. It is to be noted that hydrogen representative of a fuelgas is in contact with the fuel electrode 1, and air (oxygen) is incontact with the air electrode 2. The current collector 4 is made from amaterial having a high current collecting performance and a highstability even in an oxidizing water vapor atmosphere. A preferredmaterial of the current collector 4 is dense graphite. The surface,facing to the fuel electrode 1, of the current collector 4 has grooves 4a extending in the horizontal direction, through which hydrogen is to besupplied, and the surface, facing to the air electrode 2, of the currentcollector 4 has grooves 4 b extending in the vertical direction, throughwhich air is to be supplied.

As shown in FIG. 2, the fuel electrode 1 and the air electrode 2, in anembodiment, are arranged with the electrolyte 3 sandwiched therebetween.The fuel electrode 1 is composed of a gas diffusion electrode 1 a and acatalyst layer 1 b, and the air electrode 2 is composed of a gasdiffusion electrode 2 a and a catalyst layer 2 b. Each of the gasdiffusion electrodes 1 a and 2 a is made from a porous material, andeach of the catalyst layers 2 a and 2 b is made from a mixture of anelectrolyte and carbon particles in which an electrode catalyst such asplatinum is supported. A plurality of the above-described fuel cells arestacked as basic units to each other, to form a fuel cell. These fuelcells are connected in series, to output a specific voltage.

In the fuel cell having the above-described configuration, when hydrogengas flows in the grooves 4 a formed in the current collector 4 so as tobe in contact with the fuel electrode 1, and air (oxygen) flows in thegrooves 4 b formed in the current collector 4 so as to be in contactwith the air electrode 2, reaction occurs on each of the fuel electrode1 side and the air electrode 2 side.

The reaction on the fuel electrode 1 side is expressed by the followingreaction formula:H₂→2H⁺+2e ⁻

The reaction on the air electrode 2 side is expressed by the followingreaction formula:½O₂+2H⁺+2e ⁻→H₂O

The reaction in the whole fuel cell is expressed by the followingreaction formula:H₂+½O₂→H₂O

As shown in the above reaction formulas, hydrogen is converted intoprotons by release of electrons therefrom at the fuel electrode 1, andthe protons are migrated to the air electrode 2 side through theelectrolyte 3, to react with electrons and oxygen at the air electrode2. As a result of such electrochemical reaction, an electromotive forceis generated.

FIG. 3 shows one example of a system of operating the fuel cell composedof the fuel cells each having the above-described configurationaccording to an embodiment of the present invention. As shown in thefigure, air is supplied from an air supply unit exemplified by an airpump 11 to the air electrodes of the fuel cells 10, and hydrogenrepresentative of a fuel gas is supplied from a fuel supply unitexemplified by a hydrogen storage tank 12 to the fuel electrodes of thefuel cells 10.

The above-described fuel cell operating system includes a CPU 13 as acontrol mechanism for executing various kinds of control. For example,the air pump 11 for supplying air, which is provided for compressing airand supplying the compressed air to an air passage 14 in order to supplyoxygen required for reaction at the cathode electrodes (fuelelectrodes), is controlled by the CPU 13. That is to say, the controlvoltage applied to the air pump 11 is controlled by the CPU 13. Morespecifically, the control voltage is outputted from the CPU 13 via a D/Aconverter 15. The control voltage is then applied to a drive amplifier16 of the air pump 11, to control an amount of air to be supplied by theair pump 11.

Further, a humidity sensor 17 for monitoring a relative humidity ofexhausted air is provided on an air exhaust unit connected to the end ofthe air passage 14. The air having been used for reaction is exhaustedfrom the air exhaust unit. The output voltage of the humidity sensor 17is subjected to A/D conversion via an A/D converter 18, and theconverted output voltage is inputted in the CPU 13. A current detectingunit 20 for detecting an output current flowing in a load 19 is providedfor the fuel cell 10, and the detected current value is subjected to A/Dconversion by an A/D converter 21 and the converted current value isthen inputted in the CPU 13.

The fuel cell operating system having the above-described configurationaccording to an embodiment of the present invention provides an improvedand unique system without the need of any complicated systemconfiguration, that is, a simple, small-sized, and inexpensive systemthat is capable of effectively detecting the state of moisture producedin the fuel cell and controlling exhaust of the produced moisture. Asdescribed above, in each fuel cell, moisture is generally produced at acathode electrode by chemical reaction between oxygen and hydrogen. Themoisture may be condensed into a liquid, and the liquid tends tooverflow in an oxygen gas passage, to obstruct diffusion of oxygen gas,thereby inhibiting effective reaction. This leads to a problem inreducing the output of each fuel cell. To address such a problem, thesystem according to an embodiment of the present invention is capable ofvaporizing the moisture and exhausting the vaporized moisture to theoutside of the fuel cell as described in detail below.

First, to supply an oxygen amount proportional to an output current tothe fuel cell 10, it is required to supply at least a flow rate of air,which corresponds to the above oxygen amount, to the fuel cell 10. Forthis purpose, an output current value I flowing in the load 19 isdetected by the current detecting unit 20, being subjected to A/Dconversion by the A/D converter 21, and is inputted in the CPU 13. TheCPU 13 generates a control voltage V₀ (expressed by the followingequation) for allowing the flow of air corresponding to the outputcurrent.V ₀ =K ₁ ×I (K ₁: specific coefficient)

The above-described flow of air produced by the air pump 11 is used forvaporizing and discharging the moisture obstructing an air passage.Examples of various types of moisture present in the fuel cell mayinclude moisture produced by reaction between hydrogen and oxygen,moisture which has been contained in a humidified hydrogen side gas andhas been then permeated on the cathode side through an electrolyte filmtogether with protons, moisture originally contained in sucked air, andmoisture initially present in the form of dew condensation in a flowpassage. To detect all of these types of moisture and optimally controlthe exhaust thereof, a large scale system would be required as generallyrecognized in the art. In other words, it is generally recognized to bedifficult to detect all of these kinds of moisture and thus achieveoptimal control of the exhaust thereof by a small-sized and inexpensivesystem.

According to an embodiment of the present invention, however, thesetypes of moisture can be readily detected as a whole and thus optimalcontrol of exhaust thereof can be achieved. This can be achieved bymonitoring a relative humidity of exhausted air and estimating the totalamount of the moisture in the fuel cell based on the monitored resultaccording to an embodiment of the present invention.

In this regard, when air flows in the fuel cell in a gas-liquid phaseequilibrium state, moisture is vaporized until the vaporized amount ofmoisture reaches a saturated water vapor amount that results in anincrease in the relative humidity of exhausted air. On the other hand,when the amount of moisture in the fuel cell is small, the amount ofwater vapor is nearly equal to an amount of sucked water vapor. In thefuel cell, as described above, moisture may occur due to various causes,the amounts thereof varying depending on the conditions of the fuelcell. Also, along with the change in temperature in a fuel celldepending on the environment and/or the operational condition thereof,the saturated water vapor pressure may be significantly varied. In anyevent, the condensation state of the water vapor in the fuel cell can beestimated by monitoring a relative humidity at the temperature of thefuel cell.

Since the temperature of exhausted air and the relative humidity ofexhausted air at an air exhaust port of the fuel cell desirablycorrespond to the temperature and the humidity in the fuel cellrespectively, the moisture state in the fuel cell can be estimated bymonitoring the temperature of exhausted air and the relative humidity ofexhausted air. For example, when the relative humidity of exhaust airbecomes close to 100%, it can be understood that condensation ofmoisture begins in the fuel cell. In this way, the time point at whichcondensation of moisture in the fuel cell begins can be checked bymonitoring an exhausted air relative humidity (Rh), and therefore,condensation of moisture can be suppressed by adjusting a supply of airby the air pump 11 so as to keep the humidity of the exhausted air to aspecified humidity level or less in order to provide effective operationof the fuel cell.

In practice, a control voltage (V) (expressed by the followingequation), which is proportional to the supply of air and is to appliedto the air pump 11, is calculated by the CPU 13 so as to be associatedwith the exhausted air relative humidity (Rh) detected by the humiditysensor 17.V=K ₂ ×Rh+V ₀ (K ₂: specific coefficient)

Accordingly, when the exhausted air relative humidity (Rh) is increased,the control voltage (V), which is adjusted so as to increase the supplyof air, is supplied to the drive amplifier 16, to thereby maintain theexhaust air relative humidity at a constant and desired value.

At a low temperature, for example, at 10° C. or less, since evaporationof moisture is inactive because of a low saturated water vapor pressure,the effect of evaporating the moisture is low even by increasing thesupply of air; however, in an ordinary operational temperature range,such as from about 50° C. to about 80° C., the control of the supply ofair can have a significant effect on the moisture level in the fuelcell, thereby making it possible to effectively prevent an air passagefrom being blocked with moisture and hence to realize safe and effectiveoperation of the system while avoiding the reduction in output.

It should be appreciated that the present invention is not limited tothe above-described embodiment. For example, the present invention in anembodiment is applicable to a system of a type in which a humidifier isprovided on the hydrogen supply side to humidify hydrogen and controlthe humidified amount of hydrogen.

As is apparent from the above description, according to an embodiment ofthe present invention, it is possible to suppress condensation ofmoisture obstructing an air passage without the need of any complicatedsystem configuration, that is, only with a minimal sensor and a simplesystem configuration, and hence to realize a small-sized fuel celloperating system and a small-sized power supply system that can operateeffectively at reduced costs. Further, by using such a fuel celloperating system, the power supply unit can be safely operated withoutreduction in output thereof, according to an embodiment of the presentinvention.

It should be understood that various changes and modifications to thepresently preferred embodiments described herein will be apparent tothose skilled in the art. Such changes and modifications can be madewithout departing from the spirit and scope of the present subjectmatter and without diminishing its intended advantages. It is thereforeintended that such changes and modifications be covered by the appendedclaims.

1. A method of operating a fuel cell generating an electromotive forcefrom the fuel cell by supplying air and a fuel gas thereto, the methodcomprising: monitoring a relative humidity of exhausted air; andcontrolling a supply of air such that the supply of air is increased toan amount effective to decrease the relative humidity of exhaust airallowing effective operation of the fuel cell.